Metal Roofing and Home Ventilation
Do metal roofs bring special ventilation needs or condensation concerns when installed on homes? The general answer is “no” but there is more than a simple answer.
Unless a home has been built specifically in a manner which does not require attic ventilation (such as a conditioned space or hot attic, or insulated panel construction), there are 3 reasons why good ventilation is important with ANY home – regardless of the roof covering.
3 Reasons Why Good Ventilation is Important for Any Home
- Energy Efficiency. Ventilation is used during the summer to vent heat out of the attic, reducing air conditioning costs. For many years, building science experts were focused on reflectivity as a way to keep heat out of the attic. While reflective or “cool roofs” are still a good idea, tests have shown that good ventilation which quickly removes any gained heat back out of the attic is equally if not more effective at home energy efficiency.
- Condensation. A great deal of moisture is generated inside of a home’s living space. It is generated primarily by laundry, showers and bathing, houseplants, cooking, and ventless stoves. That moisture, originating in the living space of the home, migrates upward and, if a 100% reliable vapor barrier is not in place behind the home’s ceilings, it ends up in the attic. Once it’s in the attic, if that warm, moisture-laden air reaches a cool surface, it condenses. (Think of that cool glass of lemonade sitting outside on a hot, humid day.) The resulting condensation can create a very unhealthy environment prone to support mold, mildew, and other biological growth. A well-vented attic also helps to keep your attic’s insulation dry which increases its effectiveness. As it gains moisture, insulation loses considerable R-Value and effectiveness. The general calculation for this is a 2.5% decrease in R Value for every 1% increase in insulation moisture content.
- Ice Dams. For homes in northern climates, good attic ventilation can help minimize the potential for winter ice dams. Ice dams occur when warm air from the living space reaches the roof deck and causes the snow to melt. The melted snow then runs down the roof and refreezes over the home’s cold overhangs, creating damaging and dangerous ice dams which can force melted snow into the home. Proper attic ventilation ideally keeps the attic the same as outside temperatures, helping to prevent the snow load on the roof from melting due to interior heat and then forming ice dams as it reaches cold areas of the roof.
With traditional roofing choices, there is a 4th reason why attic ventilation can be helpful; good attic ventilation can help increase shingle life. By keeping rooftop temperatures a bit cooler, shingle life can be preserved. This benefit does not really have an impact on roofing materials like metal, tile, and slate which are unaffected by high temperatures.
How do you achieve good attic ventilation?
Good attic ventilation requires both intake and exhaust vents. Intake vents are at the bottom of the roof, often in the home’s overhangs. Exhaust vents are typically at or near the ridge or peak of the roof. You want to have ventilation that creates total continuous air exchanges in the attic. Outside air flows in at the bottom of the roof, travels along the underside of the roof deck, and then carries heat and moisture from the attic, out of the attic space through the exhaust vents. We think of it as continually bathing the entire underside of the roof deck with fresh air, carrying out moisture and heat.
When evaluating your attic’s ventilation, it is important to check and make sure that the soffit intake vents are not blocked by insulation. Blockages can happen over time when insulation is added to a home. Additionally, if you go the route of a ridge vent as your exhaust vent, any other exhaust vents including gable vents must be removed or blocked off. Otherwise, they will begin to act as intake vents and feed air to the ridge vent. In an attic where the air is moving properly, you will be able to observe air movement using a smoke pen or a small piece of light tissue paper.
Intake and exhaust vents
Ideally, you want a balance between intake and exhaust vents. You want equal amounts of intake and exhaust vents. If the amounts cannot be perfectly balanced then, if anything, you want slightly more intake vent than exhaust vent. This creates a pressurized system. If on the other hand, you have more exhaust vent than intake vent, you can start to bring air and weather in through the exhaust vents on the roof – not a good thing. Ventilation product manufacturers will be able to tell you the amount of ventilation offered by their products, or you can calculate it yourself. This is often called the net free airflow.
How much ventilation do you need?
In most areas, the International Residential Code calls for 1 square foot of ventilation per 150 square feet of attic floor space. As described above, the 1 square foot should be divided equally between intake and exhaust. If a home’s roof has a very steep pitch, and thus greater attic volume, the suggested amount of ventilation should be increased by around 20%. In certain extreme northern climates (Climate Zones 7 and 8), if the home has a qualifying vapor barrier behind its ceilings, ventilation can be reduced by code to as little as 1 square foot per 300 square feet of attic floor space. Our general advice, though, is that more ventilation is better than less.
Keep in mind: some structures have been designed and built to not require attic ventilation. The building codes are beginning to address these methods of construction.
A metal roof typically does not increase nor decrease the need for ventilation. Ventilation that meets code requirements is adequate for any type of roofing material, including metal. We do need to mention, though, one very rare scenario we have seen just a couple of times over our 35+ years. For existing homes in condensation-prone climates with ventilation that does not come anywhere close to meeting code standards, a metal roof can become the straw that breaks the camel’s back by decreasing the roof deck temperature just enough that dewpoint is reached in the attic, causing condensation to occur where it has not occurred before. In rare situations like this, an experienced contractor can help find a suitable solution. If you think your home might be borderline in having condensation and moisture control issues, be sure to discuss that with your contractor before starting any roofing project.
We hope that this information is helpful. If you have ventilation or roofing questions, feel free to email them to our own guru, Todd Miller, at email@example.com.
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My new Classic roof adds to the appearance of the house, and I expect it to remain the same for years to come. Lawrence Daingerfield, TX